Post on 20-Jun-2020
The Mineral Mosaic
Quartz (SiO2) is one of the most common and most important rock forming
minerals of the silica group. Quartz is typically colourless or white, like the samples used
in the display to depict the snow on the mountains. The white color is caused by fluid
inclusions of gas and/or liquid that gets trapped during crystal formation. Some other
varieties of quartz that appear in the display are agate, tiger’s eye, aventurine, and
jasper.
Agate is a banded form of chalcedony and a cryptocrystalline variety of quartz.
Cryptocrystalline means the crystal structure is only visible under magnification. The
banding associated with agate is due to deposition in cavities and is typically found
parallel to the walls of the cavity. Natural agate shows colourful banding and can also be
artificially dyed. This is easy to do because chalcedony is slightly porous and allows the
dye to be absorbed. Agate is used in the display as part of the base of the volcano as
well as the ash plume being ejected from the volcano.
Tiger’s eye is formed when blue asbestos
starts to get replaced by quartz. The tiger’s eye
can retain the asbestos’ blue colour as seen in
the mountains of the mosaic, but is more
commonly seen as a golden brown colour. This is
from the oxidization of iron and can be seen as
part of the base of the volcano in the mosaic.
Jasper is another type of chalcedony
or cryptocrystalline variety of quartz. It is an
opaque variety and is generally red, yellow or
brown. The variety in our display is red, and
this colour is due to included colloidal
particles of iron oxides.
The last variety of quartz used in the mosaic represents grass and is aventurine.
It is characterized by its translucency and a shimmering or glistening characteristic called
aventurescence. Aventurine is most commonly green, and can have fuchsite inclusions
in it. Fuchsite is a chromium-‐rich variety of muscovite and gives aventurine a banded
appearance. This can cause aventurine to be classified as a rock sometimes.
The last mineral used in the mosaic is
sodalite (Na8(AlSiO4)6Cl2). It is a commonly blue
tectosilicate mineral with a translucent
characteristic. It is most commonly found in both
volcanic and plutonic rocks of the nepheline
syenite family, and is generally associated with
nepheline.
References Berry, L. G, Brian Harold Mason, and Richard Vincent Dietrich. Mineralogy, Concepts, Descriptions, Determinations. San Francisco: Freeman, 1983. Print. All photos courtesy of The University of Waterloo Earth Sciences Museum.